Mail Gateway Node
Add role as mail gateway node only.
That node will only filter incoming/outgoing mails thru spam, antivrus, blacklists, rbls and pass thru all good mails to mail node server.
On mail gateway: spamassassin with pyzor, razor, dcc, uribl.com, rbl check + clamav or other + mhr scanner + pearl scanner
Main mail node can skip scans from mg node so it's less loaded